In the related art, Yagi-Uda antennas are one of antenna devices well known to the public. Such Yagi-Uda antennas include a planar type that employs a dielectric substrate in order to be included in a vehicle-mounted radar apparatus or the like to save space. Non-Patent Document 1 discloses an antenna device including an array of such planar Yagi-Uda antennas.
FIGS. 12(A) and (B) are configuration diagrams of an antenna disclosed in Non-Patent Document 1, whereas (C) is a configuration diagram of an array antenna in which a plurality of antenna devices of (A) and (B) are arranged. Meanwhile, illustration of a ground electrode provided on a back surface is omitted in (C).
As shown in FIG. 12, in an antenna device 100 of Non-Patent Document 1, a feeder portion electrode 20, an unbalanced-balanced transformer electrode (hereinafter, referred to as a balun electrode) 30, a radiation portion electrode 40, and a waveguide portion electrode 50 are formed on a top surface 111 of a dielectric substrate 101, whereas a ground electrode 60 is formed on a back surface 112 thereof.
The feeder portion electrode 20 is formed like a line extending in a predetermined direction. One end thereof is connected to the balun electrode 30. The balun electrode 30 has two U-shaped electrodes arranged so that openings thereof face each other and is formed in a shape spreading in a direction vertical to the extending direction of the feeder portion electrode 20. One of the two U-shaped electrodes (the U-shaped electrode on the right when FIG. 12 is viewed from the front) is formed in a shape of which the electrical length thereof is longer than that of the other one by a half wavelength (λ/2) of a transmission/reception signal. With this shape, a current path from the feeder portion electrode 20, which is an unbalanced line, to the radiation portion electrode 40, which is a balanced line, is maintained and transmission and reception signals are transferred. The radiation portion electrode 40 has two linear electrodes, having a predetermined length, extending in a direction vertical to the extending direction of the feeder portion electrode 20. The electrodes thereof are connected to the two electrodes of the balun electrode 30, respectively. This structure allows the radiation portion electrode 40 to function as a radiation portion of a dipole antenna. The waveguide portion electrode 50 is separated from the radiation portion electrode 40 by a predetermined interval and in parallel to the radiation portion electrode 40. The ground electrode 60 is formed on the back surface 112 corresponding to an area including the feeder portion electrode 20 and the balun electrode 30.
In addition, an array antenna of Non-Patent Document 1 includes antenna devices 100A-100D, each having the feeder portion electrode 20, the balun electrode 30, the radiation portion electrode 40, the waveguide portion electrode 50, and the ground electrode 60, arranged on the dielectric substrate 101 at a predetermined interval. The feeder portion electrodes of the antenna devices 100A and 100B are connected to a branch circuit 71, whereas the feeder portion electrodes of the antenna devices 100C and 100D are connected to a branch circuit 72. The branch circuits 71 and 72 are connected to a branch circuit 73. This structure allows a transmission wave signal fed to the branch circuit 73 to be diverged by the branch circuit 73 into the branch circuits 71 and 72, to be diverged by the branch circuit 71 into the antenna devices 100A and 100B, and to be diverged by the branch circuit 72 into the antenna devices 100C and 100D. On the other hand, a reflected wave signal received by the antenna devices 100A and 100B is transferred to a processing unit at a subsequent stage through the branch circuits 71 and 73. A reflected wave signal received by the antenna devices 100C and 100D is transferred to the processing unit at the subsequent stage through the branch circuits 72 and 73.    Non-Patent Document 1: William R. Deal, Noritake Kaneda, James Sor, Yongxi Qian, and Tatsuo Itoh, “A New Quasi-Yagi Antenna for Planar Active Antenna Arrays”, JUNE 2000, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 48, NO. 6.
Nevertheless, since a feeder portion and a balun portion are separately formed in an antenna device shown in FIGS. 12(A) and (B) and the balun portion includes two U-shaped electrodes spreading in a direction vertical to an extending direction of the feeder portion, the antenna device requires a certain size of space although the antenna device has already been miniaturized. In addition, when an array antenna is formed using these antenna devices as shown in FIG. 12(C), a relatively large space is needed for each antenna device. Accordingly, when the number of antennas to be arranged is increased to sharpen the directivity of a reception beam for the purpose of an improvement in the detection accuracy, the space for the feeding portion and the balun portion relative to the entire space of the array antenna increases. Thus, decreasing the space is problematic when an array antenna using a plurality of these antenna devices, a multi-sector antenna having this array antenna, and a high-frequency wave transceiver are miniaturized. In addition, since the length of a transmission line connecting each unit becomes long, a transmission loss increases and an antenna gain decreases.